The present invention relates to a process for producing dinitrotoluene (DNT) by the nitration of toluene with nitric acid under adiabatic conditions.
It is known that toluene can be nitrated by an adiabatic process to form dinitrotoluene. Toluene is reacted with nitrating acid (a mixture of sulfuric and nitric acids which has an HNO3 content of 1 to 20% by weight) while being mixed thoroughly. The hot reaction product is subjected to phase separation, water is evaporated and the concentrated spent acid is recycled to the nitration process. When the spent acid is concentrated, the water of reaction formed during nitration is expelled, together with any other water which may be present. The heat of reaction is used to expel the water.
One of the problems encountered in this process is that part of the DNT which is still dissolved in the acid passes over with the separated water. The DNT which is carried over with the steam becomes solid under the condensation conditions and covers the cooling surfaces of the condensation heat exchanger. The solidification point of typical mixtures of DNT isomers is about 55xc2x0 C. Due to the formation of such deposits, heat transfer is significantly impaired, steam can no longer be condensed to a sufficient extent, and the condenser has to be repeatedly taken out of operation and cleaned.
In the conventional xe2x80x9cisothermalxe2x80x9d two-stage process for producing DNT, the problem may be solved by injecting mononitrotoluene (MNT), which is formed and isolated in the first nitration step, into the vapor space of the evaporator (DE-A-3,409,719). The MNT which is injected in this manner reduces the melting point of the DNT and thus ensures that the vapors also remain liquid under the conditions for the condensation of water. The organic phase of the vapor condensate which is isolated by phase separation is recycled to the reactors of the dinitration stage.
This solution cannot, however, be employed in a single-stage, adiabatic dinitration of toluene process (EP-A-597,361) because an MNT stream which can be isolated does not exist in this process. MNT is formed as an intermediate in the adiabatic single-stage process, but it is immediately nitrated further to form DNT.
EP-696,569 describes a single-stage adiabatic process for the production of DNT which solves the aforementioned problem. In this disclosed process, nitration is conducted so that small amounts of mononitrotoluene are still present in the reaction mixture after nitration. The spent acid from the reaction is condensed before the separation of the organic constituents. The reaction product leaving the reactor, which still contains MNT, enters the concentration stage directly, wherein the MNT is preferentially volatilized overhead with water and DNT fractions. The amount of MNT which remains in the product mixture after reaction has to be selected so that coverage of the condensation heat exchanger by organic products does not occur during the concentration stage.
A disadvantage of the method described in EP-696,569 is that the MNT is not completely evaporated with the water in the concentration stage and a small amount of MNT residue always remains in the reaction product. This MNT constitutes a loss of yield because it is not separated from the DNT and recycled to the nitration stage. Separation is not conventionally effected until the DNT has been hydrogenated to form TDA (toluene diamine). The aminotoluene which is formed from the MNT is then separated by distillation. This results in an additional distillation cost for the hydrogenation process, as well as additional costs for hydrogen, catalyst and energy.
For these reasons, efforts are made to keep the MNT content in the reaction product as low as possible and to operate with only the minimum amount of MNT necessary to keep the vapor condenser clear. In practical operation, however, it is often problematicalxe2x80x94if not impossiblexe2x80x94to maintain limiting conditions of this type.
Moreover, the necessity of a minimum content of residual MNT limits the purity of the DNT product which can be attained.
Process control of this type (i.e., problem-free condensation of vapors together with DNT which is as free from MNT as possible) imposes very high demands on the evaporation unit. In practice, it is scarcely possible to construct and operate an evaporation unit which satisfies each of these criteria. A certain residual content of MNT in the nitration product and thus a loss in yield is ultimately unavoidable. Moreover, it is extremely difficult to ensure a residual content of MNT at the reactor outlet which is low enough that the vapor condensation stage operates effectively and is not impaired by the formation of solids.
It is an object of the present invention to provide a continuous, adiabatic process for producing dinitrotoluene.
It is also an object of the present invention to provide an adiabatic process for producing dinitrotoluene which reliably prevents the formation of solid deposits in the vapor condensation stage.
It is a further object of the present invention to provide an adiabatic process for the production of dinitrotoluene which can be conducted economically and in a technically simple manner.
These and other objects which will be apparent to those skilled in the art are accomplished by reacting toluene with nitric acid under adiabatic conditions at a temperature of from about 60 to about 200xc2x0 C. in amounts such that the molar ratio of toluene to nitric acid is from about 1:1.5 to about 1:3. The reaction mixture thus generated is then concentrated to a water content of up to 30% by weight. Dinitrotoluene (DNT) present in the reaction mixture is at least partially, if not completely, removed by conventional method either before or after concentration. A solvent is added to the vapor containing DNT which is generating during the concentration of the reaction mixture. The resultant mixture of solvent and vapor may then be recycled to the reaction vessel as a mixture. The solvent/vapor mixture may also be separated with the solvent being recycled and the DNT either combined with the product DNT or recycled to the reaction vessel.
The present invention relates to a process for producing dinitrotoluene by the adiabatic nitration of toluene with nitric acid at temperatures of from about 60 to about 200xc2x0 C. and at a molar ratio of toluene to nitric acid of from about 1:1.5 to about 1:3.0. The reaction mixture obtained is concentrated to a water content of up to about 30% by weight (based on the total weight of the concentrated mixture existing of H2O, HNO3 and H2SO4). The dinitrotoluene which is also present in the reaction mixture is completely or partially removed from the nitration reaction mixture by any of the known techniques either before or after concentration of that mixture. Any of the DNT which is still present in the vapor which is generated during the concentration of the reaction mixture is kept liquid by the addition of a solvent. The solvent added to the DNT-containing vapor is separated, together with the DNT, from the water (aqueous phase) removed during the concentration of the reaction mixture. This solvent/DNT mixture may subsequently be treated to separate the DNT from the solvent and recycle the solvent to the vapor generated during the concentration of the reaction mixture. The DNT separated from the solvent/DNT mixture may then be combined with the product DNT recovered from the reaction mixture or recycled to the nitration reaction vessel. It is also possible to recycle the DNT/solvent mixture directly to the nitration reaction vessel.
The nitration process of the present invention is preferably conducted at temperatures of from about 90 to about 180xc2x0 C., more preferably from about 95 to about 170xc2x0 C., and most preferably from about 100 to about 160xc2x0 C.
The molar ratio of toluene to nitric acid in the nitrating process of the present invention is preferably from about 1:1.7 to about 1:2.5, most preferably from about 1:1.8 to about 1:2.2.
Any of the nitrating acids known to be useful for the nitration of aromatic compounds may be used as the nitrating acid in the process of the present invention. The nitrating acid is usually a mixture of sulfuric and nitric acids generally having a nitric acid content of from about 0.5 to about 15% by weight (based on total weight of nitrating acid), preferably from about 1.5 to about 8% by weight.
In principle, any organic substance which dissolves DNT and which is capable of preventing the formation of deposits on heat exchangers is a suitable solvent to be added to the vapor generated during concentration of the nitration reaction mixture. Those organic compounds which have a boiling point of from about 80 to about 250xc2x0 C., preferably from about 100 to about 200xc2x0 C., at normal pressure are particularly suitable solvents.
Such solvents include C1-C15 hydrocarbons which may be substituted, for example, by nitro groups or halogens. Other materials which are suitable as solvents include aromatic compounds and olefins, which may be substituted by halogen, and also petroleum spirits. Specific examples of useful solvents include: toluene, xylene, chlorobenzene, dichlorobenzene, trichlorobenzene, isododecane, dodecane, and mononitrotoluene. Toluene and/or any of the mononitrotoluenes formed during the process are preferably used as solvents. The solvents can of course be used individually or in admixture with each other.
The solvent is added to the vapor or to the vapor condensate in an amount such that the ratio by weight of the total solvent used to the DNT present in the vapor or vapor condensate is from about 50:1 to about 1:10, preferably from about 20:1 to about 1:5.
It is important to the success of the process of the present invention that the solvent be added separately to the vapor and that the ratios by weight of solvent to DNT in the vapor come within the aforementioned ranges.
The reaction mixture generated by the reaction of toluene and nitric acid is concentrated to a water content of up to 30% by weight (based on the weight of the concentrated nitration mixture existing of H2O, HNO3 and H2SO4). This reaction mixture is preferably concentrated to a water content of up to 27% by weight.
The DNT which is present in the vapor or vapor condensate obtained during the concentration of the reaction mixture usually falls within a quantitative range from 2 to 50% by weight, based on the total amount of vapor.
Concentration of the reaction mixture can be carried out by any of the known techniques such as standard distillation or flash distillation. The reaction mixture which is concentrated after the separation of DNT is composed essentially of an aqueous phase of concentrated sulfuric acid and may also include residual organic constituents such as dinitrotoluene, mononitrotoluene, nitrocresols, nitrobenzoic acid or nitrosulfuric acid. This concentrated mixture is recycled to the reaction vessel into which toluene and nitrating acid are introduced.
The yield of DNT produced by the process of the present invention is xe2x89xa796% after a conventional crude DNT purification stage. The purity of the DNT obtained is xe2x89xa798%.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.